The present invention relates to methods and apparatus for regulating pressure in a laser tube of a gas laser and to methods for constructing the pressure regulating components of a gas laser. The present invention relates particularly to a gas laser of the kind having a laser tube filled with low pressure gas and subject to pressure variations which tend to interfere with or to prevent lasing operation. The method and apparatus of the invention overcome or minimize such undesirable pressure variations.
In gas laser tubes of the kind used in the present invention, lasing occurs with a selected gas, such as, for example, argon, krypton or xenon. These gas lasers operate at low tube pressures of a few torr, and the pressure in the tube is important for proper operation.
Most gas lasers, however, are subject to unpredictable tube pressure variations for a number of reasons. U.S. Pat. No. 4,477,908 issued Oct. 16, 1984 and assigned to the same assignee as the assignee of this application discusses some of the reasons for such pressure fluctuations. The gas within the laser tube may be lost due to ions of the gas being buried in the gas tube cathode. Ions may be entrapped by sputtering effects within the tube. The gas ions may subsequently be released to again form free gas within the laser tube. These factors can cause undesirable pressure variations.
A gas laser tube has an anode and a cathode. The anode and cathode are spaced apart and develop a voltage which is related to the pressure of the gas within the tube. That voltage level is also important in connection with the operating characteristics of the laser.
Pressure variations and related variations of voltage may interfere with the proper operation of the laser and may also cause substantial problems in connection with the power supply producing the voltage within the tube. If substantial pressure and voltage variations occur within the tube, it may be necessary for the power supply to be oversized with respect to the tube in order to assure proper control of the voltage and current characteristics with the tube. Close regulation of the pressure and voltage within the tube permits the use of smaller, more efficient and more economical power supplies.
Pressure variation and instability within gas lasers become even more severe in lasers using heavy gases such as krypton. Krypton lasers are desirable in a number of applications because of their ability to function effectively at ultraviolet, visible and infrared wave lengths.
Window problems become more severe in lasers covering a wide range of wavelengths. Crystalline quartz is a good window material because it has a wide transmission range and minimizes solarization or darkening after prolonged exposure to radiation. Crystalline quartz windows, however, do have a relatively high affinity for dirt and other foreign material which may be present within the gas tube.
Mechanical components, including storage tanks, valves and pumps, have been employed for regulating the pressure in systems designed and intended to avoid spontaneous pressure variations. These mechanical components have seals formed from plastic or other organic material. Even if the seals are formed from polyfluorotetraethylene materials (available, for example, under the TEFLON trademark of the du Pont Company) organic particles or gases tend to be introduced into the system and to find their way into the gas tube. The organic particles and gases are attracted to crystalline quartz and form monomolecular layers on the surfaces of the windows. These monomolecular layers of dirt and other organic foreign material increasingly interfere with operation of the laser apparatus, particularly over extended periods of time.
Avoiding or minimizing pressure variations within the gas tubes has been a problem in the prior art.
Avoiding the deposition of dirt and organic films on windows, particularly upon crystalline quartz windows, has been another problem in the prior art.
In arriving at the present invention, other techniques were tested as a means for avoiding these problems. A gas ballast reservoir was connected to the tube with the objective that relatively limited pressure fluctuations within the tube would be minimized by the large volume of gas in the ballast. The use of a ballast appeared desirable because it avoided the use of mechanical pumps and eliminated seals which would tend to introduce foreign material. The use of a ballast, however, did not satisfactorily overcome all the problems.
The presence of the molecular sieve materials used for pressure control not only avoids the introduction of organic materials to control pressure, but provides a means of absorbing organics introduced inadvertently during the manufacturing process.